Pharmacokinetic factors can assist greatly in both dose-setting for toxicity studies and in interpretation of the results. Chemicals on-test by the NTP are nominated for disposition studies. The absorption (oral, dermal), distribution, metabolism, and excretion is studied in rats and other species as needed. The effect of dose is determined. In this way, the effects of chronic exposure may be predicted. The first chemicals to be studied in this project include o-benzyl-p-chlorophenol (BCP), and citral (oil of lemon). The dermal absorption of 4,4'-thiobis-(6-t-butyl-m-cresol) (TBBC) was also studied in rats and mice.